Recently, electronic devices using a CPU and sensors have been mounted on many vehicles to meet the social needs for advanced intelligence and safety for automobiles. In particular, a wheel speed sensor has been used as a basic sensor for enhancing various controls, intelligence and safety for vehicles. A device for detecting the wheel speed that uses the wheel speed sensor must use a filter. The frequency characteristics of a low-pass filter for processing analog signals must be precisely determined. Therefore, it is important to precisely set a circuit constant that is determined by a capacitor and a resistor. To decrease the size, the device for detecting the wheel speed must be realized in the form of an IC. Among the circuit constants in the filter, the resistors can be formed in the form of a MOS integrated circuit in small sizes, but the capacitors cannot be highly precisely constituted in the MOS integrated circuit. Moreover, their sizes cannot be decreased as that of the resistors. In the conventional devices, therefore, capacitors having precise capacitances are attached as external parts of an IC to solve the problem of precision in the filter circuit constant. A device for detecting the wheel speed having the above circuit constitution has been disclosed in JP-A-2000-2553.
In the above device for detecting the wheel speed, too, however, capacitors have been attached to the outer side for determining the constant of the filter circuit. Therefore, the device is not integrated in a perfect all-in-one form, and the size cannot be decreased any more. Moreover, the capacitors attached to the outer side cause an increase in the number of places of physical connection such as soldering. This affects reliability and makes it difficult to decrease the cost since the capacitors attached to the outer side must have highly precise capacities.